The invention relates to electrical circuits for driving individual picture elements of an electronic display, particularly, an Organic Light Emitting Device (OLED) display.
Organic light emitting devices have been known for approximately two decades. OLEDs work on certain general principles. An OLED is typically a laminate formed on a substrate such as soda-lime glass or silicon. A light-emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between a cathode and an anode. The light-emitting layer may be selected from any of a multitude of luminescent organic solids, and may consist of multiple sublayers or a single blended layer of such material. The cathode may be constructed of a low work function material while the anode may be constructed from a high work function material. Either the OLED anode or the cathode (or both) should be transparent in order to allow the emitted light to pass through to the viewer. The semiconductor layers may include hole-injecting or electron-injecting layers.
When a potential difference is applied across the device (from cathode to anode), negatively charged electrons move from the cathode to the electron-injecting layer and finally into the layer(s) of organic material. At the same time positive charges, typically referred to as holes, move from the anode to the hole-injecting layer and finally into the same organic light-emitting layer(s). When the positive and negative charges meet in the organic material, they produce photons.
The wave lengthxe2x80x94and consequently the colorxe2x80x94of the photons depends on the material properties of the organic material in which the photons are generated. The color of light emitted from the OLED can be controlled by the selection of the organic material, or by the selection of dopants, or by other techniques known in the art. Different colored light may be generated by mixing the emitted light from different OLEDs. For example, white light may be produced by mixing the light from blue, red, and yellow OLEDs simultaneously.
In a matrix-addressed OLED device, numerous individual OLEDs may be formed on a single substrate and arranged in groups in a grid pattern. Several OLED groups forming a column of the grid may share a common cathode, or cathode line. Several OLED groups forming a row of the grid may share a common anode, or anode line. The individual OLEDs in a given group emit light when their cathode line and anode line are activated at the same time. A group of OLEDs within the matrix may form one pixel in a display, with each OLED usually serving as one subpixel or pixel cell.
OLEDs have a number of beneficial characteristics. These include: a low activation voltage (about 5 volts); fast response when formed with a thin light-emitting layer; high brightness in proportion to the injected electric current; high visibility due to self-emission; superior impact resistance; and ease of handling. OLEDs have practical application in television, graphic display systems, and digital printing. Although substantial progress has been made in the development of OLEDs to date, additional challenges remain.
For example, OLED brightness may be controlled by adjusting the current or voltage supplied to the anode and cathode. Light output for an OLED driven by current, however, may be more stable than for a voltage driven OLED, and thus, current driven devices are preferred. For applications such as microdisplays where the pixel size can be very small (a few microns on the side), the resulting current requirement may be very small, typically a few nanoamperes.
The requirement for small driving currents is further complicated by the need for gray scale control. The relative amount of light generated by an OLED is commonly referred to as the xe2x80x9cgray scalexe2x80x9d or xe2x80x9cgray level.xe2x80x9d Acceptable gray scale response (as seen by the human eye) requires a constant ratio between adjacent gray levels. In a current driven device, gray scale control is achieved by controlling the amount of current applied to the OLED. Application of the power law shows that very small driving currents are required in order to obtain the darker levels of the gray scale. Accomplishing gray scale control in microdisplays, such as those referenced above, may be particularly difficult due to the inherently small driving currents called for in microdisplays in the first instance. In some cases the required currents may be as low as a tens of pico-amperes, depending on the organic material luminous efficiency and the pixel size. Such current levels are on the same level as the leakage currents encountered in conventional cmos processes. It is therefore extremely difficult, if not impractical, to successfully control gray scale in microdisplays by varying current magnitude.
The challenge is further compounded by two major factors: Transistor transconductance function and transistor to transistor variability over the IC area.
An additional challenge is presented by active matrix OLEDs that are addressed on a row-by-row basis. It is important in row addressed OLEDs that the correct driving signal reach the destination pixel no matter where the pixel is located (i.e. without regard to whether the pixel is located at the beginning or the end of the row being addressed). Thus, settling time may be an issue. The preferred method of transporting a driving signal with a reduced settling time impact is to use a voltage source with a low output impedance. Since the OLED requires current, the voltage must be transformed into a current. A mos transistor may be used to achieve this transformation. The mos transistor may be tied to a capacitor used to store the voltage used in the transformation. The voltage to current transfer function for the transistor is proportional to the square of the gate-source voltage. Accordingly, as the current required to achieve particular levels of gray scale decreases, the voltage stored on the capacitor tied to the gate electrode decreases even more rapidly. This relationship makes it increasingly difficult to generate the small voltages required for the lower gray scale levels. Furthermore, the voltage-current transformation relationship makes it difficult to convey the correct driving signal without it being derogated by ambient noise. Still further, the need for low level currents translates into a need for longer channel lengths for the current source transistor, which may place a constraint on pixel size.
The production of OLEDs heavily involves semiconductor processing. Semiconductor processes inherently produce some non-uniformities in the OLEDs produced. These non-uniformities may produce threshold voltage variations in the finished device. Because the operation of a current driven OLED leads to the current source transistor operating near its threshold voltage, such variations can have an adverse effect on display uniformity. This situation may worsen as the current requirement is decreased, such that the non-uniformity effect dominates (and thus degrades) the gray scale performance of the display.
References that illustrate the difficulty of addressing the aforementioned challenges include a U.S. patent issued to Ching Tang of the Eastman Kodak Corporation that describes a two transistor and storage capacitor structure. The structure described in Tang has exhibits the problems mentioned above. Another relevant reference that is illustrative of the aforementioned challenges is a U.S. patent issued to Dawson et al. of Sarnoff Research Laboratories. This patent is aimed at solving the threshold voltage variation encountered with poly-crystalline silicon processes, but does not address the small current limitations nor the need for a small control voltage. Finally it requires additional devices and places a lower limit on pixel sizes.
The present innovation introduces a second control signal and changes the operation of the current source from a linear mode to a switched mode. By relying on the switched mode, the current source can be designed and optimized for the maximum current required, as opposed to needing to be able to provide all the current values needed. Use of a switched mode of operation removes or largely reduces the challenges associated with very small current values and drastically reduces the impact of leakage currents.
Furthermore, the switched mode of operation (also called pulse width modulation) allows for the use of larger voltage values at the storage element, thus improving the design margins. The larger voltage values enabled by this technique reduce the effect of threshold voltage variations, as well as the susceptibility to noise created by switching control signals.
Finally, the switched mode allows an effective turn-off of the current source and thus provides for the required uniform black level for the display.
It is an object of the present invention to provide a circuit and method for driving organic light emitting display pixels.
It is another object of the present invention to provide a current driver for a display pixel, wherein the current driver is controlled by a pulse-width modulated voltage.
It is still another object of the present invention to provide a method of driving a display pixel using a current source.
It is yet another object of the present invention to provide a circuit and method for improving gray scale control of an organic light emitting display.
It is still another object of the present invention to provide a circuit and method for improving gray scale uniformity of an organic light emitting display.
It is still yet another object of the present invention to provide a circuit and method for increasing the operational life of an organic light emitting display.
It is still a further object of the present invention to provide a circuit and method for controlling the luminance of a display without substantially affecting the contrast ratio of the display.
It is still a further object of the present invention to provide a circuit and method for reducing the impact of leakage currents experienced in driving an OLED.
It is still another object of the present invention to provide a circuit and method for driving an OLED in which the driving circuit may be designed and optimized for a maximum driving current.
It is yet another object of the present invention to provide a circuit and method for driving an OLED that reduces the effect of threshold voltage variations.
It is still yet another object of the present invention to provide a circuit and method for driving an OLED that provides a uniform display black level.
Additional objects and advantages of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.
In response to this challenge, Applicants have developed an innovative circuit for driving a light emitting diode in a display using a current supply, said circuit comprising: a first transistor having a source, a drain, and a gate; a current supply connected to the first transistor source; an anode terminal of a light emitting diode connected to the first transistor drain; and a means for applying a combination of at least two voltages to the first transistor gate so as to control the time that the current supply is connected to the light emitting diode.
Applicants have also developed a method of driving a light emitting diode in a display using a current supply, said method comprising the steps of: applying current to an OLED responsive to at least one power transistor being in a turned on state; turning on at least one access transistor responsive to a cyclical voltage; applying a DATA voltage to a node responsive to the access transistor being turned on, said node being connected to at least the access transistor, a capacitor, and the at least one power transistor; charging the capacitor responsive to the application of the DATA voltage to the node; turning off the at least one access transistor so as to discontinue charging the capacitor in response to the DATA voltage; applying a cyclical variable amplitude voltage to the capacitor; further charging the capacitor responsive to the application of the cyclical variable amplitude voltage to the capacitor; and turning the at least one power transistor off responsive to the voltage at the node so as to selectively control the current supplied to the OLED from a current source.
Applicants have also developed a method of driving a light emitting diode in a display using a current supply, said method comprising the steps of: applying current to an OLED responsive to at least one power transistor being in a turned on state; selectively turning the at least one power transistor off responsive to a power transistor gate voltage comprised of the combination of a selectively set cyclical DATA voltage and a cyclical variable amplitude RAMP voltage.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated herein by reference, and which constitute a part of this specification, illustrate certain embodiments of the invention and, together with the detailed description, serve to explain the principles of the present invention.